Temperature responsive switches have been employed as flame sensing elements in appliances such as gas burners and the like. Typically, these devices have employed a bimetallic assembly of two metallic members having dissimilar coefficients of thermal expansion. The members are interconnected at one end and, at their opposite ends, are coupled to a housing and a switch mechanism within the housing whereby the differential thermal expansion of the assembly moves the blade of the switch mechanism. The switch is in an electrical circuit supplying power from a supplemental source to the solenoid of a valve and the like in the gas line to the burner.
Another type of actuator for a solenoid valve used a thermocouple in which two members of dissimilar metals exhibiting a thermoelectric effect are joined at a hot junction which is positioned adjacent the burner flame. The members' cold junctions are located at a point removed from the flame. This actuator is effective to generate a sufficient electrical voltage that can be applied to typical solenoid valves without the use of supplemental electrical sources.
A difficulty with the thermocouple actuator is that it is not rapidly responsive to extinguishment of the burner flame, often requiring one to several minutes for adequate cooling to reduce its potential sufficiently for movement of the valve closure member. A major disadvantage of the thermally responsive switch mechanism is, of course, its dependency on a supplemental electrical source.